In the 1970s, several drillings at the bottom of the Mediterranean revealed the existence of enormous salt deposits, some up to three kilometers thick! The dating of these evaporitic deposits, between 5.97 and 5.33 million years ago, will thus make it possible to affirm that the Mediterranean experienced a major hydrological crisis at the end of the Miocene, during the Messinian stage. The formation of these salt deposits implies that the sea has become considerably concentrated in salt, to the point of becoming a real brine.
Witnesses of a drastic fall in sea level
Since then, multiple studies have looked at what we call the Crisis of salinitysalinity Messinian. And it turns out that this episode is correlated with quite a few other observations made in the Mediterranean basin: sudden and brutal loss of biodiversity, aerial erosion of the margins, digging of deep fluvial canyons… All in all then suggests that this crisis was associated with a significant drop in sea level.
Tectonic reconstructions then made it possible to explain its origin: the rise of the African continent towards the north would have led to the closure of the Strait of Gibraltar around 6 million years ago, isolating the Mediterranean Sea from the Atlantic Ocean. Evaporation would then have done its work, gradually lowering sea level and allowing its salt concentration to increase.
Quantifying the extent of sea level drop
A scenario which seems well put together, but which retains gray areas. If we know that the sea level has fallen, the quantificationquantification the extent of this phenomenon remained to be determined. Because the formation of evaporites (salt and gypsumgypsum) does not necessarily require total evaporation of water. These mineralsminerals can in fact crystallize from water supersaturated with salt. Their presence, even in very large quantities (scientists estimate that the Messinian evaporitic deposits represent 1 million km3), therefore does not give any indication of the real level of the Mediterranean around 5.6 million years ago.
A team of researchers led by Giovanni Aloisi (CNRS, Institut de Physique du Globe de Paris), however, found a way to estimate this drop. Scientists have in fact used isotopesisotopes of chlorinechlorine as water level markers. Chlorine (Cl) is part of the composition of salt, but certain heavier isotopes, such as 37Cl, will precipitate preferentially. The evolution of the ratio between 37Cl and 35Cl over time in the salt deposits thus made it possible, thanks to a mathematical model, to specify the scenario of the Messinian Salinity Crisis. It is featured in an article recently published in the journal Nature communications.
A loss of 70% of the overall volume of water
The results indeed suggest that this hydrological crisis was divided into two stages. The first, between 5.97 and 5.6 million years ago, would not have been associated with a significant drop in sea level. It seems that exchanges with the Atlantic Ocean continued during this period, making it possible to maintain the global level but without succeeding in stabilizing the chemical balance of the sea, which would have started to become saturated with salt.
We actually see the first deposits appearing during this period. The total isolation of the Mediterranean basin would have occurred around 5.6 million years ago. Cut off from the water supply from the Atlantic, the Mediterranean would then have started to dry up quickly. In just 10 million years, it would have lost 70% of its volumevolume overall. In places, the drop could have reached 1.7 to 2.1 kilometers! A hydrological situation which led to the massive deposition of evaporites. It is likely that large areas of the seabed have been completely dried up, allowing the colonization of certain islands, such as the Balearics, by new speciesspecies. The loss of such an important massemasse of water is also in agreement with certain geological observations, which show the triggering of volcanic eruptions in the basin during this second phase, due to the rapid decompression of the terrestrial mantleterrestrial mantle.
This crisis, however, had an end. The results of the article clearly show a rapid re-filling of the basin 5.33 million years ago.
An observation which corroborates the idea of a rupture of the Gibraltar threshold and the sudden spilling of waters from the Atlantic into the Mediterranean.
